(1) Field of the Invention
This invention is in the field of methods by which each module of a local area network (LAN) satisfying IEEE Standard 802.4 and having redundant cables over which cables the modules of the LAN communicate, detects a fault in a cable from which the module is receiving signals and switches to the redundant cable to receive signals from it.
(2) Description of the Prior Art
The Institute of Electrical and Electronics Engineers, Inc. (IEEE) has established a standard for Token Passing Local Area Networks, IEEE 802.4. An advantage of a standardized, or universal, token passing LAN result is that users of such a LAN can connect equipment to the LAN supplied by any vendor whose equipment satisfies the IEEE Standard 802.4.
There are presently commercially available devices such as Motorola Inc.'s, MC 68824, "Token Bus Controllers," which implement the Media Access Control (MAC) portion of IEEE 802.4. IEEE 802.4 defines the physical and MAC portions of the data link layer standards of the Manufacturing Automation Protocol (MAP) specification. Also commercially available are Concord Communication, Inc.'s "Carrierband Modular Modems" which are also designed to meet the requirements of IEEE 802.4.
When a LAN is utilized to control an industrial process, reliability of the LAN is a high priority requirement. Reliability of a LAN can be significantly increased by redundancy of the modules, or nodes, of the LAN and the media, over which the modules communicate, such as a coaxial cable.
However, the IEEE 802.4 standard for a token passing LAN is satisfied by a single bus medium such as a bidirectional coaxial cable so that problems associated with the use of redundant cables are not present in such a LAN. Commercially available modems satisfying IEEE 802.4 such as the Concord's "Carrierband Modular Modems" have the capability of transmitting simultaneously over two parallel bidirectional coaxial cables, but they can receive signals from only one of the two cables at any one time. Thus the nodes, or modules, of a token passing LAN satisfying IEEE 802.4 using commercially available token bus controllers (TBC) and modems do not have the capability to determine when either, or both of the redundant cables has a fault, indicated by no signals being received for a predetermined period of time over one of the cables or by the fact that the line or cable is too noisy, so that valid signals, or messages, are not being received properly; and, thus, when a module should switch to the redundant cable.